59th Annual Report on Research 2014

Oil water interfaces have a wide variety of important applications ranging from the understanding of oil spills to enhanced oil recovery. As crude oil is a highly complex mixture it is informative to use well controlled model mixtures to address this problem in detail. Currently we focus on simulations of salt water with the lighter components of crude. The most abundant light components are alkanes and aromatics. As these types of molecules are strongly different in their polarity it is not surprising that there is evidence for interface induced sorting such that the aromatics localize closer to the water phase than the aliphatics. Crude oil has therefore also significantly lower surface tension than pure alkanes of the same density. We are studying in this project currently this effect in detail and for the first time calculate the ensuing surface tension using a detailed molecular model.

Simulation Details

We have performed detailed Molecular Dynamics simulations with the Gromacs 4.5 package and the CHARMM27 force—field under NpT conditions. The molecules are arranged in slab geometry, i.e. we have a flat oil water interface. The current model systems contain heptane as prototype alkane and toluene as aromatic.

Tab 1. Simulations of fresh and salt water

Molecular type	Number of molecules
	Fresh 1	Salt 1	Fresh 2	Salt 2
HEP	500	500	444	444
TOL	694	694	750	750
SPE	8449	8269	7241	7087
Na+	-	90	-	77
Cl-	-	90	-	77

System	Interfacial Tensions (mN/m)
	Calculated	MD (Lit.)	Expt’l (Lit.)
Pure Heptane	51.90	51.96	51.9, 50.2
Pure Toluene	36.97	37.69	36.0, 36.4, 36.1
Fresh: conc. 1	42.68 +/- 0.39	42.27	40
Salt: conc. 1	42.86 +/- 0.22
Fresh: conc. 2	41.87 +/- 0.21
Salt: conc. 2	42.84 +/- 0.55

We find that the surface tension is only weekly affected by adding salt.

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Fig 1: Left Orientation of Toluene at the interface, Right Orientation of water at the interface.

In Fig 1 we see that toluene at the interface likes to lie flat for both geometrical reasons and to optimize the weak electrostatic interactions with water. This is in agreement with our earlier results that toluene enriches at the interface. Water also orients itself at the interface such that its dipole moment right at the interface is normal to the interface again optimizing the electrostatic interactions with the enriched toluene.

Fig 2: Density profile in salt water/oil systems

Additionally we find (Fig 2) that salt is excluded from the interface as the hydrogen bonding structure of the water is severely disrupted making it hard for water to provide an appropriate shell of hydration close to the oil.

We also performed a number of coarse-grained simulations

Figure 3: A coarse-grained simulation of oil and water interface.

In fig 3 The blue and cyan particles represent water and antifreeze beads. The purple and brown particles represent benzene and octane molecules, respectively. The oil phase is represented by a mixture of molecules containing 50 mol percent benzene and 50 mol percent octane. Each coarse-grained blue particle represents 4 water molecules respectively. The linear shape of Octane is roughly modeled with two coarse-grains bonded together, whereas the planar shape of benzene is represented by three coarse-grains in a triangle shape. Clearly, in the above picture we observe the aggregation of aromatic rings at the oil-water surface. The size of the simulation box is 20 nm x 20 nm x 13 nm with temperature of 300K and pressure of 1 bar maintained using the Berendsen thermostat and barostat. The above conformation is observed after 50 ns in simulation time. Approximately ten percent of the blue water beads are replaced antifreeze particles in order to prevent the water phase from becoming frozen.

Figure 4: Lateral density profile of coarse-grained oil-water interface.

Also in the CG simulations (fig 4) we see the enrichment of aromatics at the interface. “OCT” and “BENZ” represent the octane and benzene profiles respectively, whereas the “W” and “WF” data are for the water and antifreeze coarse-grained beads. The accumulation of benzene at the two oil-water interfaces can be clearly seen. The total density of the aqueous phase is greater than that of the organic model oil mixture.